The present invention relates to a method for traction slip control in automotive vehicles with the aid of wheel brakes which are actuated by pressure fluid. The wheel brakes are connected for the development of braking pressure to an auxiliary-pressure source for specific intervals derived from the rotational behavior of the wheels.
The use of the brake system of the automotive vehicle for the control of traction slip is known. To this end, an anti-lock control system including an auxiliary-pressure source and sensors for measurement of the wheel rotational behavior is provided such that, even in the event of the brake not applied, namely in the presence of excessive driving force or for control of traction slip, braking pressure can be built up in the driven wheels by way of additional pressure fluid conduits. (German laid-open applications Nos. 33 38 826; 34 07 538; and, 34 07 539). With the aid of electromagnetically actuated multidirectional control valves, a connection is established between the pressure source and the wheel brake of the skidding wheel, a braking pressure generator or directly. The magnitude of the necessary braking energy is determined from the wheel rotational behavior by means of calculating circuits in the electronic control unit and is predefined by the duration of the intervals of excitation, that is the open-passage periods, of the valve which constitutes the connection between the pressure source and the wheel brake. The quality of the control is dependent on the precision of pressure proportioning. The pulses or pulse trains determining the open-passage periods of the valve therefore must be set precisely. This is particularly so because these valves are wheel brakes which are connected to the auxiliary-pressure source and thus to a very high pressure level. In doing so, difficulties arise with the known methods and brake systems. This is because, at the beginning of brake actuation, first the so-called clearance and other lost travels must be overcome. That is, the brakes must be applied prior to a braking effect being able to occur. However, the volume of pressure-transmitting medium required to cover these lost travels differs as a consequence of manufacturing tolerances, adjustment of the brakes uneven wear, etc. Accordingly, a filling period or duration of change-over of the braking-pressure inlet valves which is invariably preset in time does not permit an even pressure level in the individual wheel brakes.
To overcome these lost travels, in actual practice a filling pulse invariable in time is arranged which allows changing the braking-pressure inlet valves over to their opened position for a specific constant time. An even pressure level in the wheel brakes cannot be accomplished thereby. Varying pressure is instead built up by the constant filling pulse, depending on the lost travel covered. Proportioning of the pressure during the subsequent traction slip control operation suffers from this initial inaccuracy.
Therefore, it is an object of the present invention to overcome the difficulties described and, within traction slip control, to eliminate the effects the different lost travels which must be covered until application of the brake have on the proportioning of the braking pressure and thus on the control accuracy. This is attained by simple means and without entailing any appreciable additional effort.